Catalyst Chicago reports that the union took the action after disclosing that the district had suggested slashing teachers' salaries by 7 percent. The teachers are seeking formal mediation, according to the charge filed with the Illinois Educational Labor Relations Board.

The union contends that the district has refused at least three formal requests for mediation. Union President Karen Lewis it is too early to talk about a staging a strike, as teachers did in 2012, because the union must go through several steps before it gets to that point.

"The financial crises facing Chicago Public Schools has arrived at our classroom doors and threatens to destroy our top priority--ensuring our students and teachers have the resources they need to succeed in the classroom. While CPS has not reviewed the complaint and is unable to comment specifically on its content, we look forward to continuing our conversations on the next contract as well as joining teachers, principals and parents to work together to increase state funding for education and reform a broken system that forces Chicago residents to pay twice for teacher pensions."